1. Field of the Invention
The present invention relates to an electro-optical device typified by an EL (electroluminescence) display device formed by fabricating semiconductor elements (elements using a semiconductor thin film, typically thin film transistors) on the surface of a substrate, and an electronic device (electronic equipment) including the electrooptical device as a display. Particularly, the invention relates to a method of manufacturing the same.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (hereinafter referred to as a xe2x80x9cTFTxe2x80x9d) on a substrate has made remarkable progress, and its application and development to an active matrix type display device has proceeded. Especially, since a TFT using a polysilicon film has a field effect mobility higher than a conventional TFT using an amorphous silicon film, a high speed operation can be made. Thus, it becomes possible to control a pixel, which has been conventionally controlled by a driving circuit external to a substrate, by a driving circuit formed on the same substrate as the pixel.
Attention has been paid to such an active matrix type display device since various merits, such as reduction of manufacturing cost, miniaturization of a display device, increase of yield, and reduction of throughput, can be obtained by forming various circuits and elements on the same substrate.
In the active matrix type EL display device, a switching element made of a TFT is provided for each pixel, and a driving element for making current control is operated by the switching element, so that an EL layer (light emitting layer) is made to emit light. For example, there is an EL display device disclosed in U.S. Pat. No. 5,684,365 (see Japanese Patent Application Laid-open No. Hei 8-234683), or Japanese Patent Application Laid-open Publication No. Hei 10-189252.
As a method of forming the EL layer, various methods have been proposed. For example, there can be enumerated vacuum evaporation, sputtering, spin coating, roll coating, a cast method, LB method, ion plating, a dipping method, an inkjet method, and the like.
An object of the present invention is to reduce a manufacturing cost of an EL layer and to provide an inexpensive EL display device. Another object of the invention is to reduce a product cost of an electronic device (electronic equipment) including the EL display device as a display.
In order to achieve the foregoing objects, the present invention is characterized in that an EL layer is formed by printing. A relief printing or a screen printing can be used as the printing method, and a relief printing is specifically preferable. The case of using a relief printing in the present invention is described here by using FIG. 1.
Shown in FIGS. 1A to 1C are a part of a relief printing apparatus used in the present invention. In FIGS. 1A to 1C, reference numeral 110 denotes an anilox roll; 111, a doctor bar (also referred to as a doctor blade); mixture of an EL element and a solvent (hereinafter referred to as EL forming substance) 112 is pooled at around the surface of the anilox roll 110 by the doctor bar 111. Note that the EL material referred here is a fluorescent organic compound and denotes an organic compound which is referred to a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer or an electron injection layer in general.
Meshed grooves (hereinafter referred to as mesh) 110a are provided on the surface of the anilox roll 110 as shown in FIG. 1B, and the mesh 110a holds the EL forming substance 112 on its surface by rolling in a direction of arrow A. Note that the dotted line shown in the figure means that the EL forming substance is held on the surface of the anilox roll 110.
Reference numeral 113 is a printing roll and 114, a relief, and unevenness is formed on the surface of the relief 114 by etching, etc. Such state is shown in FIG. 1C. In the case of FIG. 1C, patterns for the pixel section 114a are formed in plural parts on the relief 114 in order to manufacture a plurality of EL display devices over one substrate. Further, projections 114b are formed at positions corresponding to a plurality of pixels when a pattern for the pixel section 114a is enlarged.
The above stated anilox roll 110 keeps the EL forming substance 112 on the mesh 110a by rolling. On the other hand, the printing roll 113 turns in a direction of an arrow B and only the projections 114b of the relief 114 contact the mesh 110a. Here, EL forming substance 112 is coated on the surface of the projections 114b. 
EL forming substance 112 is printed at the sections where the projections 114b and a substrate 115 that is shifted in horizontal direction (direction of an arrow C) at the same speed as the printing roll 113 contacts. By doing so, EL forming substance 112 is printed on the substrate 115 in a state of arrangement into a matrix.
EL material is then resided by evaporating the solvent included in the EL forming substance 112 through heat treatment in a vacuum. It is therefore necessary to use a solvent that evaporates at a lower temperature than the glass transition temperature (Tg) of the EL material. The thickness of the finally formed EL layer is determined by the viscosity of the EL forming substance. In this case the velocity can be controlled by selection of the solvent, and the velocity of 10 to 50 cp (preferably 20 to 30 cp) is preferable.
Further, the possibility of crystallizing the EL material through evaporation of solvent becomes high when much impurities that can be a crystalline nucleus exist in the EL forming substance 112. The crystallization reduces the light emitting efficiency and thus it is not preferable. It is preferable that least possible impurity is contained in the EL forming substance 112.
It is important to make the environment clean as possible, at refining the solvent, refining the EL material or mixing the solvent and the EL material for reducing the impurities, and it is also preferable to pay attention to the atmosphere on printing the EL forming substance by the printing apparatus of FIG. 1. In concrete, it is preferable to perform the above stated printing process of the EL forming substance by a printing apparatus placed in a clean booth filled with an inert gas such as nitrogen.
Note that the present invention can be implemented to both of an active matrix EL display device and a passive matrix (simple matrix) EL display device.